A Rail Builder is a civilian unit that becomes available part-way through a game of Imperialism 2, as soon as you research Early Steam Engine.

The following description is from dminoz's "Daily Imperialist". (Authorship and copyright not certain, and his e-mail seems to be dead; if he or anyone else claiming authorship thinks this should not be here, let us know.) :

The Railroad Builder is one the two civilian units that aren't available right from the start of the game (on higher difficulty settings, the Merchant is the other). You can recruit Railroad Builders once you've researched Early Steam Engine.
In contrast to the Engineer and the ordinary, soft-hatted Builder, the Railroad Builder does only one thing: he builds railways with great abandon until you tell him to stop or there's no way to build any more. Because each section of rail is expensive, it's a good idea to check on his progress every turn.
If you're playing at Normal level or higher, the very fact that you got to see a Railroad Builder in action means that you've having a good game. It doesn't tend to happen before the early 18th century. It's a very turbulent time, with plenty of new developments taking place, so make sure you give your proposed new railway the attention it deserves. Remember that a railway can only be built on top of an existing road.

The building stops when the Rail Builder reaches a road junction; he needs you to tell him which way to go.

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